1. Field of the Invention
The present invention relates to a medical image generating apparatus that generates a three-dimensional image used in diagnostic imaging, imaging diagnostic system using the apparatus, medical image generating method and medical image generating program, and particularly to a medical image generating apparatus, which is suitable for diagnosis of an invasive abnormal region, and system, method and program.
2. Description of the Related Art
Conventionally, there is used imaging diagnostic in which a tomographic image of an interior of a human body is imaged by an imaging device (modality) such as a CT (Computed Tomography) scanner, an MRI (Magnetic Resonance Imaging) and the like and the result is used in diagnosis of disease. Also, such a method is established that a three-dimensional image of such as a predetermined internal organ, an organ and the like is generated from the obtained tomographic image to make a diagnosis. This method is contributed to a precise and accurate diagnosis.
In order to display the internal organ of the interior of the human body three-dimensionally, volume rendering is performed using a method, which is generally called “ray casting.” Herein, the volume rendering means that a three-dimensional image is directly drawn from digital data of an object obtained by CT. In this “ray casting”, a virtual light ray is projected to a voxel, which indicates a minute three-dimensional structural unit of an object. Then, reflected lights of all voxels where the light ray reaches are calculated and a pixel value (including, for example, a RGB value that is color information) of each pixel is obtained to generate a three-dimensional image. Moreover, as disclosed in Unexamined Japanese Patent KOKAI Publication No. 2002-312809, such a method is established that a gradient, which is obtained based on voxel values around the arrival point of the virtual light ray to draw a shadow on a surface and display a shape of the surface clearly.
FIG. 9 illustrates an example that an intestinal wall surface of an intestinal tract is displayed as a three-dimensional image by such method. As illustrated in the figure, since the shape of the surface is clearly displayed by a shading effect based on the gradient, for example, a polyp (regions shown by arrows in the figure) developed on the surface of the intestinal wall is clearly displayed. Accordingly, the diagnosis using the three-dimensional image generated by the aforementioned method is useful in discovering an abnormal region developed in the interior of the human body.
However, in the ray casting using the gradient, though the abnormal region on the surface can be clearly displayed, for example, an abnormal region developed invasively in the interior of tissue cannot be displayed. FIG. 10 shows an example in which an intestinal tract where an invasive abnormal region develops is displayed as a three-dimensional image using the aforementioned method. As illustrated in the figure, the invasive abnormal region cannot be confirmed on a screen.
In order to solve this problem, there is used a method in which an opacity value of the surface portion is reduced to display a portion lower than the surface at the time of performing volume rendering. FIG. 11 illustrates an example of a three-dimensional image generated by this method. As illustrated in this figure, the invasive abnormal region is distinguishably displayed in a gray scale (a region shown by an arrow in the figure). However, as recognized by comparison between FIG. 11 and FIG. 10, according to this method, drawing of the surface portion becomes unclear and the shape of the surface is not clearly displayed. In other words, though the invasive abnormal region is clearly displayed, there occurs such trouble that the polyp and the like on the surface are not displayed. Accordingly, for example, there is need to generate multiple images based on the volume rendering of voxels each having a different opacity value. Or, the opacity value of the voxel must be changed according to the position of the presence of the abnormal region to be displayed at the time of executing the volume rendering. Namely, complicated working and much time are required, preventing a rapid diagnosis.